There are only a limited number of gene regulatory elements such as promoters, 5′ untranslated region (5′ UTR) and 3′ untranslated region (3′ UTR) available for plastid transgene expression and most of them are plastid sequences. As plastid genomes are highly active in homologous recombination, the insertion in the genome of endogeneous sequences as regulatory element could bring about genomic rearrangements resulting in loss or inactivation of the transgenic function. In order to prevent such genomic rearrangement, foreign sequences sharing little homology with plastid genomic DNA sequence should be used as regulatory elements for plastid transgene expression.
In land plant plastids, the mRNA 5′ UTR sequences are essential for mRNA stability and translation initiation process. The 5′ UTRs of most highly expressed plastid genes contain a Shine-Dalgarno like sequence that is complimentary to the 3′ end of the plastid 16S rRNA and is believed to play a predominant role in translation initiation. It is possible that foreign sequences that contain a Shine-Dalgarno like sequence might be able to function as plastid gene translation element. The bacteriophage T7 gene 10 5′ UTR sequence, which contains a SD element, was previously shown to be very efficient in promoting translation in plastids (McBride et al., (1994) Proc. Natl. Acad. Sci. 91: 7301–7305; Ye et al., (2001) Plant J. 25: 261–270; Kuroda and Maliga (2001) Nucl. Acids Res. 29: 970–975).